This invention relates to improved photopolymer compositions suitable for use in the manufacture of flexographic photopolymer printing plates that are processed in aqueous media. The use of photopolymer compositions that are processed in organic solvents or in aqueous media for the manufacture of flexographic photopolymer printing plates is known, but existing photopolymer compositions processable in aqueous media have undesirable characteristics.
Compositions useful for the manufacture of flexographic photopolymer printing plates are generally made by blending a flexible polymer, an appropriate crosslinking agent, and a photoinitiator to allow room temperature reaction of the polymer with the crosslinking agent upon exposure of the blend to actinic radiation. Small amounts of other chemicals such as antioxidants, antiozonants, surfactants or dyes, are sometimes added.
"Actinic radiation" as used herein refers to radiation that initiates a chemical reaction and is intended to include ultraviolet radiation, electromagnetic wavelengths other than the ultraviolet and non-electromagnetic radiation such as electron beams. The use of actinic radiation to initiate a crosslinking reaction does not exclude the use of additional initiation methods, such as heat. Generally, flexographic photopolymer printing plates are photoinitiated using ultraviolet (UV) radiation.
In the flexographic photopolymer process, selected areas of the photopolymer are subjected to actinic radiation thereby crosslinking and insolubilizing them and then the image is developed by contacting the exposed sheet with a solvent which permeates the unreacted polymeric portion and causes it to swell and weaken. The reacted polymeric portions of the sheet being crosslinked remain unaffected because only a small amount of solvent can permeate those portions. The image made of reacted portions is thus revealed by removing at least part of the unreacted polymeric portion of the photopolymer sheet by contact with moving brushes or other means exposing the image. Typically, substantially all the uncrosslinked polymeric portion is removed. Then the developed sheet is removed from the development chamber and dried. Additional processing can occur. For example, the plate can receive further exposure to actinic radiation to insure complete crosslinking of any remaining unreacted components. The process for the manufacture of prior art flexographic printing plates is described in more detail in U.S. Pat. No. 4,369,246 to Chen.
The use of organic solvents for the processing of photopolymer plates (i.e., removal of unreacted polymer), however, has the drawback that environmentally sensitive solvents are typically used. These organic solvents are often flammable, relatively hazardous, and are highly regulated by most industrialized nations.
There is thus a need for the development of novel photopolymer compositions that can be processed in aqueous media to eliminate the use of organic solvents while maintaining the benefits of flexographic photopolymer printing plates processed in organic solvents.
The recent interest in this area of research and development has led to several attempts at aqueous processable photopolymers as described in, for example, U.S. Pat. No. 5,364,741 to Huynh-Tran, U.S. Pat. No. 4,517,279 to Worns, U.S. Pat. No. 4,177,074 to Proskow, and U.S. Pat. No. 4,198,241 to Bronstert. However, these "aqueous" photopolymer systems have a number of undesirable features especially a reliance on environmental sensitive components in their preferred developing solutions to achieve an acceptable processing rate. The preferred development solutions for these "aqueous" photopolymer systems also have a pH far from neutral. The Huynh-Tran system has a relatively low pH (pH of 2.4) and the Worns and Proskow systems have a relatively high pH (pH of 12.3). None of these "aqueous" photopolymer systems has effectively eliminated the use of environmentally sensitive or corrosive chemicals in the processing solutions.
Thus, there is a need for a water processable photopolymer composition, with acceptable processing characteristics and with physical properties suitable for use in flexographic photopolymer printing plate applications that can be processed in aqueous solutions that neither require the use of environmentally sensitive chemicals, nor extremely high or low pH.